Transmission device for differential communication

ABSTRACT

In a transmission device for differential communication, a first cathode-side element part is coupled between a first communication line and a cathode-side power supply line, a second cathode-side element part is coupled between a second communication line and the cathode-side power supply line, a first anode-side element part is coupled between the first communication line and an anode-side power supply line, and a second anode-side element part is coupled between the second communication line and the anode-side power supply line. A driving portion drives the element parts based on transmission data input from an outside. A target potential generating portion generates target potentials of the element parts based on potentials of the first communication line and the second communication line.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is based on and claims priority to JapanesePatent Application No. 2009-275623 filed on Dec. 3, 2009, the contentsof which are incorporated in their entirety herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a transmission device for differentialcommunication.

2. Description of the Related Art

In a conventional transmission device for differential communication, apower source voltage is applied to a pair of communication lines througha transistor having a small output impedance, a, polarity of the appliedvoltage is inverted based on a value of transmission data, and therebythe transmission data is transmitted by a differential transmissionmethod.

If a common mode noise having the same phase is superposed to each ofthe communication lines and potentials of the communication linesincrease or decrease, electric current flows in the communication linesso that the potentials are maintained within a range of the power sourcevoltage and a differential output is maintained.

However, the above-described operation cannot deal with a common modenoise that is much larger than the power source voltage. For example, US2007/0252659 A (corresponding to WO2006/040869) discloses a differentialcommunication system in which a common mode choke coil is provided inseries with communication lines and a common mode noise is reduced bythe common mode choke coil.

Because the common mode choke coil includes a magnetic substance coreand a winding, the common mode choke coil is difficult to be included inan integrated circuit with a transmitting circuit or a receivingcircuit. Thus, when a common mode choke coil is used in a differentialcommunication system for restricting a failure due to a common modenoise, there is a difficulty that a substrate area may be increased anda dimension of a transmitting device or a receiving device may beincreased and that a cost of a transmitting device or a receiving devicemay be increased.

SUMMARY OF THE INVENTION

In view of the foregoing problems, it is an object of the presentinvention to provide a transmission device for differentialcommunication that can transmit transmission data at a predetermineddifferential voltage without using a common mode choke coil even when acommon mode noise exceeding a range of a power source voltage issuperposed into communication lines.

A transmission device for differential communication according to anaspect of the present invention includes a first communication line, asecond communication line, a cathode-side power supply line, ananode-side power supply line, a first cathode-side element part, asecond cathode-side element part, a first anode-side element part, asecond anode-side element part, a driving portion, and a targetpotential generating portion. The cathode-side power supply line iscoupled to a cathode side of a direct current power source. Theanode-side power supply line is coupled to an anode side of the directcurrent power source. The first cathode-side element part is coupledbetween the first communication line and the cathode-side power supplyline. The second cathode-side element part is coupled between the secondcommunication line and the cathode-side power supply line. The firstanode-side element part is coupled between the first communication lineand the anode-side power supply line. The second anode-side element partis coupled between the second communication line and the anode-sidepower supply line. The driving portion applies a differential voltagebetween the first communication line and the second communication lineby driving one of a group of the first cathode-side element part and thesecond anode-side element part and a group of the second cathode-sideelement part and the first anode-side element part based on a value oftransmission data input from an outside, and thereby transmitting thetransmission data by a differential transmission method. The targetpotential generating portion generates at least one of a group of afirst target potential and a second target potential and a group of athird target potential and a fourth target potential. The first targetpotential is higher than a potential of the first communication line bya constant voltage. The second target potential is higher than apotential of the second communication line by the constant voltage. Thethird target potential is lower than the potential of the firstcommunication line by the constant voltage. The fourth target potentialis lower than the potential of the second communication line by theconstant voltage. When the driving portion drives the first cathode-sideelement part and the second anode-side element part, the driving portiongenerates the differential voltage between the first communication lineand the second communication line based on the constant voltage by atleast one of inputting the second target potential to a control terminalof the first cathode-side element part and inputting the third targetpotential to a control terminal of the second anode-side element part.When the driving portion drives the second cathode-side element part andthe first anode-side element part, the driving portion generates thedifferential voltage between the first communication line and the secondcommunication line based on the constant voltage by at least one ofinputting the first target potential to a control terminal of the secondcathode-side element part and inputting the fourth target potential to acontrol terminal of the first anode-side element part. The differentialvoltage generated when the driving portion drives the first cathode-sideelement part and the second anode-side element part and the differentialvoltage generated when the driving portion drives the secondcathode-side element part and the first anode-side element part havedifferent polarities.

In the transmission device, even when a potential of a common mode noisesuperposed to each of the communication lines is higher than a potentialof the cathode-side power supply line or lower than a potential of theanode-side power supply line, a predetermined differential voltage canbe generated between the communication lines.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects and advantages of the present invention will be morereadily apparent from the following detailed description of preferredembodiments when taken together with the accompanying drawings. In thedrawings:

FIG. 1 is a circuit diagram showing a transmission device fordifferential communication according to a first embodiment of thepresent invention;

FIG. 2 is a circuit diagram showing an exemplary configuration of thetransmission device according to the first embodiment;

FIG. 3 is a timing diagram showing a simulation result of differentialvoltages at a time when a common mode noise occurs;

FIG. 4 is a diagram showing a differential communication system used forsimulating the differential voltages shown in FIG. 3; and

FIG. 5 is a diagram showing a transmission device for differentialcommunication according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A transmission device for differential communication according to afirst embodiment of the present invention will be described withreference to FIG. 1.

The transmission device includes a pair of communication lines L1 and L2coupled with external communication lines through output terminals To1and To2, respectively. The transmission device applies a differentialvoltage, whose polarity changes in accordance with a value oftransmission data, between the communication lines L1 and L2, and thetransmission data is transmitted through the external communicationlines by a differential transmission method. For example, thetransmission device can be used in a differential communication systemused in a place, such as a vehicle, where there are many noises.

As shown in FIG. 1, the communication line L1 is coupled with a sourceof a field effect transistor (FET) Q1 whose drain is coupled with apower source line through a diode D1. The communication line L2 iscoupled with a source of a FET Q2 whose drain is coupled with the powersource line through a diode D2. The communication line L1 is alsocoupled with a source of a FET Q3 whose drain is coupled with a groundline through a diode D3. The communication line L2 is also coupled witha source of a FET Q4 whose drain is coupled with the ground line througha diode D4.

The power source line is a cathode-side power supply line coupled with acathode side of a direct-current power source. Anodes of the diodes D1and D2 are coupled with the power source line and cathodes of the diodesD1 and D2 are coupled with the drains of the FETs Q1 and Q2,respectively. Each of the FETs Q1 and Q2 is formed of an N channelMOSFET. By coupling the sources of the FETs Q1 and Q2 with thecommunication line L1 and L2, respectively, source follower circuits areformed.

Thus, when the FETs Q1 and Q2 are activated, potentials of thecommunication lines L1 and L2 are maintained at values lower than gatevoltages of the FETs Q1 and Q2 by threshold voltages of the FETs Q1 andQ2, respectively.

The ground line is an anode-side power supply line coupled with an anodeside of the direct-current power source. Cathodes of the diodes D3 andD4 are coupled with the ground line, and anodes of the FETs Q3 and Q4are coupled with the drains of the FETs Q3 and Q4. Each of the FETs Q3and Q4 is formed of a P channel MOSFET. By coupling the sources of theFETs Q3 and Q4 with the communication line L1 and L2, respectively,source follower circuits are formed.

Thus, when the FETs Q3 and Q4 are activated, the potentials of thecommunication lines L1 and L2 are maintained at values higher than gatevoltages of the FETs Q3 and Q4 by threshold voltages of the FETs Q3 andQ4, respectively.

The transmission device further includes voltage sources 11-14. Thevoltage source 11 generates a target potential V1 that is higher thanthe potential of the communication line L2 by a constant voltage Vc. Thevoltage source 12 generates a target potential V2 that is higher thanthe potential of the communication line L1 by the constant voltage Vc.The voltage source 13 generates a target potential V3 that is lower thanthe potential of the communication line L2 by the constant voltage Vc.The voltage source 14 generates a target potential V4 that is lower thanthe potential of the communication line L1 by the constant voltage Vc.

Gates of the FETs Q1 and Q4 are coupled with driving switches 21 and 24,respectively. When the value of the transmission data input throughinput terminals Ti1 and Ti2 is 1, that is, when an input signal level isa high level, the driving switches 21 and 24 are turned on and input thetarget potentials V1 and V4 to the gates of the FETs Q1 and Q4 so as toactivate the FETs Q1 and Q4. The gates of the FETs Q1 and Q4 are alsocoupled with driving stop switches 31 and 34, respectively. When thevalue of the transmission data is 0, that is, when the input signallevel is a low level, the driving stop switches 31 and 34 are turned onand couple the gates and the sources of the FETs Q1 and Q4 so as todeactivate the FETs Q1 and Q4.

Gates of the FETs Q2 and Q3 are coupled with driving switches 22 and 23,respectively. When the value of the transmission data is 0, that is,when the input signal level is the low level, the driving switches 22and 23 are turned on and input the target potentials V2 and V3 to thegates of the FETs Q2 and Q3 so as to activate the FETs Q2 and Q3. Thegates of the FETs Q2 and Q3 are also coupled with driving stop switches32 and 33, respectively. When the value of the transmission data is 1,that is, when the input signal level is the high level, the driving stopswitches 32 and 33 are turned on and couple the gates and the sources ofthe FETs Q2 and Q3 so as to deactivate the FETs Q2 and Q3.

Each of the driving switches 21-24 and the driving stop switches 31-34determines whether the value of the transmission data is 1 (high level)or 0 (low level) by comparing the signal level of the transmission datainput through the input terminals Ti1 and Ti2 with a determinationvoltage Vth.

Specifically, each of the driving switches 21-14 and the driving stopswitches 31-34 determines the value of the transmission data based on adetermination result of whether the signal level is higher than thedetermination voltage Vth.

Thus, the determination voltage Vth is a voltage between the signallevel (high level) at a time when the value of the transmission data is1 and the signal level (low level) at a time when the value of thetransmission data is 0.

In the transmission device, when the value of the transmission data 1,the FETs Q1 and Q4 are activate and the FETs Q2 and Q3 are deactivated.In contrast, when the value of transmission data is 0, the FETs Q2 andQ3 are, activated and the FETs Q1 and Q4 are deactivated.

Thus, when the transmission data is 1, that is, when the FETs Q1 and Q4are activated, the potential of the communication line L1 is thedifference when the threshold voltage of the FET Q1 is subtracted fromthe target potential V1, and the potential of the communication line L2is the sum of the target potential V4 and the threshold voltage of theFET Q4, and the potential difference between the communication lines L1and L2 is a constant voltage (+Vc−the threshold voltage).

When the transmission data is 0, that is, when the FETs Q2 and Q3 areactivated, the potential of the communication line L1 is the sum of thetarget potential V3 and the threshold voltage of the FET Q3, and thepotential of the communication line L2 is the difference when thethreshold voltage of the FET Q2 is subtracted from the target potentialV2, and the potential difference between the communication lines L1 andL2 is a constant voltage (−Vc+the threshold voltage).

Even when a common mode noise higher than a power source potential issuperposed to the external communication line coupled with the outputterminals To1 and To2 and the potentials of the communication lines L1and L2 increase or even when a common mode noise lower than a groundline potential is superposed to the external communication line coupledwith the output terminals To1 and To2 and the potentials of thecommunication lines L1 and L2 decrease, the potential difference betweenthe communication lines L1 and L2 is maintained at the constant voltage(+Vc−the threshold voltage or −Vc+the threshold voltage) based on thevalue of the transmission data.

The transmission device according to the present embodiment can transmitthe transmission data at a substantially constant voltage withoutproviding a common mode choke coil in the external communication linecoupled with the output terminals To1 and To2 and can secure atransmission performance of the transmission data.

In addition, because it is not necessary to provide a common mode chokecoil to the external communication line, a dimension of the differentialcommunication system can be reduced and a cost of the differentialcommunication system can be reduced. In the transmission device shown inFIG. 1, the FET Q1 and the diode D1 can function as a first cathode-sideelement part, and the FET Q2 and the diode D2 can function as a secondcathode-side element part. The FET Q3 and the diode D3 can function as afirst anode-side element part, and the FET Q4 and the diode D4 canfunction as a second anode-side element part. The driving switches 21-24and the driving stop switches 31-34 can function as a driving portion.The voltage sources 11-14 can function as a target potential generatingportion. The target potential V1 can function as a second targetpotential. The target potential V2 can function as a first targetpotential. The target potential V3 can function as a fourth targetpotential. The target potential V4 can function as a third targetpotential.

The voltage sources 11-14 shown in FIG. 1 can be provided by an externalpower source. However, in order to correspond to a case where a commonmode noise higher than the power source voltage Vd is superposed to thecommunication lines L1 and L2, the power source voltage of the externalpower source needs to be set to a high voltage. Thus, the voltagesources 11-14 may be formed of a current mirror circuit.

Each of the driving switches 21-24 and the driving stop switches 31-34may be any switch circuit that can change an on-off state based on thedata of the transmission data. For example, each of the driving switches21-24 and the driving stop switches 31-34 can be simply formed of aswitching element such as a FET and a bipolar transistor.

As an example of a configuration of the transmission device according tothe present embodiment, a case where the source voltages 11-14 areformed of a current mirror circuit and each of the driving switches21-24 and the driving stop switches 31-34 is formed of a FET will bedescribed with reference to FIG. 2.

When the voltage sources 11-14 are formed of a current mirror circuit,two constant current circuits that output a constant current using thepower source voltage Vd are formed. One of the constant current circuitsis formed of a P channel MOSFET Q41. A source of the P channel MOSFETQ41 is coupled with the power source line. A drain of the P channelMOSFET Q41 is coupled with the ground line through a resistor R41. Agate and the drain of the P channel MOSFET Q41 are directly coupled witheach other.

The other one of the constant current circuits is formed of an N channelMOSFET Q51. A source of the N channel MOSFET Q51 is coupled with theground line. A drain of the N channel MOSFET Q51 is coupled with thepower source line through a resistor R51. A gate and the drain of the Nchannel MOSFET Q41 are directly coupled with each other.

The FET Q41 is coupled with the FETs Q42 and Q43 whose gates are coupledwith each other so as to form a current mirror circuit. The FET Q51 iscoupled with the FETs Q52 and Q53 whose gates are coupled with eachother so as to form a current mirror circuit. Between the FETs Q42 andQ52, diodes D11 and D13 are coupled so that the constant current flowsfrom the FET Q42 to FET Q52. Between the FETs Q43 and Q53, diodes D12and D14 are coupled so that the constant current flows from the FET Q43to the FET Q53.

Between the diodes D11 and D13, resistors R11 and R13 are coupled.Between the diodes D12 and D14, resistors R12 and R14 are coupled. Theresistors R11-R14 have the same resistance. The communication line L2 iscoupled to a junction point of the resistors R11 and R13. Thecommunication line L1 is coupled to a junction point of the resistorsR12 and R14.

The constant current flows to each of the resistors R11-R14, and thetarget potentials V1-V4 are generated on opposite side of the resistorsR11-R14 from the communication line L1 or L2. The target potentialsV1-V4 are higher or lower than the potential of the communication lineL1 or L2 by the constant voltage Vc. The constant voltage Vc=theresistance of each of the resistors R11-R14×the constant current value.

In a case where a common mode noise higher than the power sourcepotential is superposed to each of the communication lines L1 and L2,the diodes D1 and D2 receive reverse biases. Thus, an electric currentthat flows from the communication lines L1 and L2 to the power sourcethrough the FETs Q1 and Q2 is not generated, and the target potentialsV3 and V4 lower than the potentials of the communication lines L1 and L2by the constant voltage Vc are generated. When the value of thetransmission data is 1, that is, when the FETs Q1 and Q4 are activated,the potential of the communication line L1 remains the potential of thecommon mode noise, and the potential of the communication line L2becomes the sum of the target potential V4 and the threshold voltage ofthe FET Q4. Thus, the potential difference between the communicationlines L1 and L2 is controlled to be the constant voltage (+Vc−thethreshold voltage).

When the value of the transmission data is 0, that is, when the FETs Q2and Q3 are activated, the potential of the communication line L1 is thesum of the target potential V3 and the threshold voltage of the FET Q3and the potential of the communication line L2 remains the potential ofthe common mode noise. Thus, the potential difference between thecommunication lines L1 and L2 is controlled to be the constant voltage(−Vc+the threshold voltage).

In a case where a common mode noise lower than the potential of theground line is superposed to each of the communication lines L1 and L2,the diodes D3 and D4 receive reverse biases. Thus, an electric currentthat flows from the ground line to the communication line through theFETs Q3 and Q4 is not generated, and the target potentials V1 and V2higher than the potentials of the communication lines L1 and L2 by theconstant voltage Vc are generated. When the value of the transmissiondata is 1, that is, when the FETs Q1 and Q4 are activated, the potentialof the communication line L1 is the difference when the thresholdvoltage of the FET Q1 is subtracted from the target potential V1, andthe potential of the communication line L2 remains the potential of thecommon mode noise. Thus, the potential difference between thecommunication lines L1 and L2 is controlled to be the constant voltage(+Vc−the threshold voltage).

When the transmission data is 0, that is, when the FETs Q2 and Q3 areactivated, the potential of the communication line L1 remains thepotential of the common mode noise, and the potential of thecommunication line L2 is the difference when the threshold voltage ofthe FET Q2 is subtracted from the target potential V2, and the potentialdifference between the communication lines L1 and L2 is controlled to bethe constant voltage (−Vc+the threshold voltage).

As described above, when the voltage sources 11-14 that can function asthe target potential generating portion is configurated by the currentmirror circuit, the target potentials V1-V4 higher or lower than thepotential of the communication line L1 or L2 by the constant voltage Vcare generated by supplying the constant current to the resistorsR11-R14. Thus, it is not necessary to provide an external power sourcethat can generate a high voltage in order to generate the targetpotentials V1-V4. Even when a common mode noise higher than the powersource potential is superposed to the external communication linecoupled with the output terminals To1 and To2 and the potentials of thecommunication lines L1 and L2 increase or even when a common mode noiselower than the ground line potential is superposed to the externalcommunication line coupled with the output terminals To1 and To2 and thepotentials of the communication lines L1 and L2 decrease, the potentialdifference between the communication lines L1 and L2 is maintained atthe constant voltage (+Vc−the threshold voltage or −Vc+the thresholdvoltage) based on the value of the transmission data.

When each of the driving switches 21-24 and the driving stop switches31-34 is configurated by a FET, as shown in FIG. 2, an inverting line isprovided in addition to a signal line. In the signal line, an inputsignal (transmission data) input through the input terminals Ti1 and Ti2flows as it is. In the inverting line, the signal level of the inputsignal, that is, a logic of the transmission data is inverted by aninverting circuit (NOT) and the inverted input signal flows.

The driving switches 21, 22 and the driving stop switches 31, 32 areformed of P channel MOSFETs Q21, Q22, Q31, and Q32, respectively.Sources of the FETs Q21 and Q31 are coupled with the gate of the FET Q1and are coupled with the power source line through a resistor R1.Sources of the FETs Q22 and Q32 are coupled with the gate of the FET Q2and are coupled with the power source line through a resistor R2.

The driving switches 23, 24 and the driving stop switches 33, 32 areformed of N channel MOSFETs Q23, Q24, Q33, and Q34, respectively.Sources of the FETs Q23 and Q33 are coupled with the gate of the FET Q3and are coupled with the ground line through a resistor R3. Sources ofthe FETs Q24 and Q34 are coupled with the gate of the FET Q4 and arecoupled with the ground line through a resistor R4.

Drains of the FETs Q21-Q24 are respectively coupled with ends of theresistors R11-R14 where the target potentials V1-V4 are generated.Drains of the FETs Q31-Q34 are respectively coupled with the sources ofthe FETs Q1-Q4.

Gates of the FETs Q21, Q23, Q32, and Q34 are coupled with the invertingline in which the inverted input signal (transmission data) flows. Gatesof the FETs Q31, Q32, Q22, and Q24 are coupled with the signal line inwhich the input signal flows as it is.

In the transmission device in FIG. 2, when the value of the transmissiondata is 1, the FETs Q21, Q24, Q32, Q33 are activated, the targetpotentials V1 and V4 are respectively input to the gates of the FETs Q1and Q4, the FETs Q1 and Q4 are activated, and the FETs Q2 and Q3 aredeactivated. When the value of the transmission data is 0, the FETs Q22,Q23, Q31, and Q34 are activated, the target potentials V2 and V3 arerespectively input to the gates of the FETs Q2 and Q3, the FETs Q2 andQ3 are activated, and the FETs Q1 ad Q4 are deactivated.

Thus, even when the transmission device shown in FIG. 1 is formed usingthe current circuit and the FETs Q21-Q24 and Q31-Q34 as shown in FIG. 2,the transmission device can operate as described above and can haveabove-described effects.

Next, differential voltages obtained at a transmitting end and areceiving end are simulated in a case where a common node noise higherthan the power source voltage Vd is superposed to a transmission channel(external communication line) when the transmission data is transmittedby a differential transmission method using the transmission deviceaccording to the present embodiment.

In the simulation, as shown in FIG. 4, a pseudo transmission channelincluding coils and capacitors is coupled with the output terminals To1and To2 of the transmission device according to the present embodiment.The transmission data is inverted at predetermined intervals and thetransmission data is transmitted from the transmission device. Inaddition, a common mode noise is applied to the pseudo transmissionchannel 10.

In the simulation, voltages EV of the respective output terminals To1and To2, a differential voltage DV1 between the output terminals To1 andTo2, and a differential voltage DV2 at an end of the pseudo transmissionchannel 10 are obtained as shown in FIG. 3. The power source voltage Vdis 5V.

As shown in FIG. 3, the voltages EV of the respective output terminalsTo1 and To2 change in the same phase due to the common mode noiseapplied to the pseudo transmission channel 10. However, the differentialvoltage DV1 between the output terminals To1 and To2 changes based onthe value of the transmission data without being influenced by thechange in the voltages EV. The differential voltage DV2 at the end ofthe pseud transmission channel 10 changes in response to the differentiavoltage DV1 without being influenced by the common mode noise.

The simulation result confirms that the transmission device according tothe present embodiment can normally transmit the transmission data bythe differential transmission method even when the common mode noisehigher than the power source voltage Vd is superposed to thecommunication lines.

The transmission device shown in FIG. 4 shows the transmission device inFIG. 1 in which the voltage sources 11-14 are formed of a current mirrorcircuit. That is, a configuration of the transmission device shown inFIG. 4 is a combination of the transmission devices shown in FIG. 1 andFIG. 2. Therefore, a detail description of the transmission device shownin FIG. 4 is omitted.

Second Embodiment

A transmission device according to a second embodiment of the presentinvention will be described with reference to FIG. 5.

In the transmission device according to the present embodiment, FETsQ61-Q64 are respectively coupled in parallel with the FETs Q1-Q4 thatcan function as the first cathode-side element part, the secondcathode-side element part, the first anode-side element part, and thesecond anode-side element part. When a common mode noise of a positivepolarity or negative polarity exceeding the power source voltage Vd issuperposed to the communication lines L1 and L2, electric current flowsbetween the communication lines and the power source line or between theground line and the communication lines through diodes D61-D64 so thatthe potentials of the communication lines do not exceed the range of thepower source voltage Vd. The other parts of the transmission deviceaccording to the present embodiment are similar to those of thetransmission device shown in FIG. 2.

Each of the FETs Q61 and Q62 are formed of a P channel MOSFET. A sourceand a gate of the FET Q61 are respectively coupled with the source andthe gate of the FET Q1, and a drain of the FET Q61 is coupled with thepower source line through the diode D61. A source and a gate of the FETQ62 are respectively coupled with the source and the gate of the FET Q2,and a drain of the FET Q62 is coupled with the power source line throughthe diode D62. An anode of the diode D61 is coupled with the drain ofthe FET Q61 and a cathode of the diode D61 is coupled with the powersource line. An anode of the diode D62 is coupled with the drain of theFET Q62 and a cathode of the diode D62 is coupled with the power sourceline.

Each of the FETs Q63 and Q64 are formed of an N channel MOSFET. A sourceand a gate of the FET Q63 are respectively coupled with the source andthe gate of the FET Q3, and a drain of the FET Q63 is coupled with theground line through the diode D63. A source and a gate of the FET Q64are respectively coupled with the source and the gate of the FET Q4, anda drain of the FET Q64 is coupled with the ground line through the diodeD64. A cathode of the diode D63 is coupled with the drain of the FET Q63and an anode of the diode D63 is coupled with the ground line. A cathodeof the diode D64 is coupled with the drain of the FET Q64 and a cathodeof the diode D64 is coupled with the ground line.

When a common mode noise higher than the potential of the power sourceline is superposed to the communication lines L1 and L2 and when theFETs Q1 and Q2 are activated, gate potentials of the FETs Q61 and Q62are drawn to the power source potential side by the resistors R1 and R2,and the gate potentials of the FETs Q61 and Q62 become lower than sourcepotentials of the FETs Q61 and Q62. Then, the FETs Q61 and Q62 areactivated, electric current flows from the communication lines L1 and L2to the power source line through the diodes D61 and D62, and thereby thepotentials of the communication lines L1 and L2 are respectivelycontrolled to be lower than or equal to upper limit potentials that arelower than the potential of the power source line by forward voltages ofthe diodes D61 ad D62. When the FETs Q1 and Q2 are deactivated, that is,when the FETs Q31 and Q32 are activated, the gate potential and thesource potential of each of the FETs Q61 and Q62 are the same potential.Thus, the FETs Q61 and Q62 are deactivated.

When a common mode noise of negative polarity lower than the potentialof the power source line is superposed to the communication lines L1 andL2 and when the FETs Q3 and Q4 are activated, gate potentials of theFETs Q63 and Q64 are drawn to the ground line potential side by theresistors R3 and R4, and the gate potentials of the FETs Q63 and Q64become higher than source potentials of the FETs Q63 and Q64. Then, theFETs Q63 and Q64 are activated, electric current flows from the groundline to the communication lines L1 and L2 through the diodes D61 andD62, and thereby the potentials of the communication lines L1 and L2 arerespectively controlled to be higher than or equal to lower limitpotentials that are higher than the potential of the power source lineby forward voltages of the diodes D63 ad D64. When the FETs Q3 and Q4are deactivated, that is, when the FETs Q33 and Q34 are activated, thegate potential and the source potential of each of the FETs Q63 and Q64are the same potential. Thus, the FETs Q63 and Q64 are deactivated.

The transmission device according to the present embodiment can controlthe differential voltage between the communication lines L1 and L2 to bea constant voltage based on the value of the transmission data.Furthermore, the transmission device according to the present embodimentcan restrict the potential of each of the communication lines L1 and L2from being largely away from the range of the power source voltage Vddue to a common mode noise. Thus, a receiving end can receive thetransmission data with certainty.

The FET Q61 and the diode D61 can function as a first upper-limitcontrol portion. The FET Q62 and the diode D62 can function as a secondupper-limit control portion. The FET Q63 and the diode D63 can functionas a first lower-limit control portion. The FET Q64 and the diode D64can function as a second lower-limit control portion.

Other Embodiments

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications will become apparent to those skilled in the art.

In each of the above-described embodiments, each of the firstcathode-side element part, the second cathode-side element part, thefirst anode-side element part, and the second anode-side element part isformed of a source follower circuit including an N channel MOSFET or a Pchannel MOSFET. Each of the element parts may also be formed of anemitter follower circuit that includes an NPN bipolar transistor or aPNP bipolar transistor whose emitter is coupled with corresponding oneof the communication lines L1 and L2.

In each of the above-described embodiments, the target potentials V2 andV1 higher than the potentials of the communication lines L1 and L2 bythe constant voltage Vc and the target potentials V4 and V3 lower thanthe potentials of the communication lines L1 and L2 by the constantvoltage Vc are generated with reference to the potentials of thecommunication lines L1 and L2. Then, based on the value of thetransmission data, the target potentials V1 and V4 are applied to thegates of the FETs Q1 and Q4 or the target potentials V2 and V3 areapplied to the gates of the FETs Q2 and Q3, and thereby a predetermineddifferential voltage based on the constant voltage Vc is generatedbetween the communication lines L1 and L2. Alternatively, the targetpotentials V1 and V2 or the target potentials V3 and V4 may begenerated, and the generated target potentials V1 and V2 or V3 and V4may be applied to corresponding gates of the FETs Q1 and Q2 or the FETsQ3 and Q4 so that a differential voltage is generated between thecommunication lines L1 and L2.

In this case, even when a common mode noise lower than the groundpotential is superposed to the communication lines L1 and L2 or evenwhen a common mode noise higher than the potential of the cathode-sidepower source line is superposed to the communication line L1 and L2, thedifferential communication can be normally performed. Therefore, thetransmission device may be used under a condition where a potential of acommon mode noise superposed to the communication lines L1 and L2 is oneof positive and negative.

1. A transmission device for differential communication, comprising: afirst communication line and a second communication line; a cathode-sidepower supply line coupled to a cathode side of a direct current powersource; an anode-side power supply line coupled to an anode side of thedirect current power source; a first cathode-side element part coupledbetween the first communication line and the cathode-side power supplyline; a second cathode-side element part coupled between the secondcommunication line and the cathode-side power supply line; a firstanode-side element part coupled between the first communication line andthe anode-side power supply line; a second anode-side element partcoupled between the second communication line and the anode-side powersupply line; a driving portion applying a differential voltage betweenthe first communication line and the second communication line bydriving one of a group of the first cathode-side element part and thesecond anode-side element part and a group of the second cathode-sideelement part and the first anode-side element part based on a value oftransmission data input from an outside, and thereby transmitting thetransmission data by a differential transmission method; and a targetpotential generating portion generating at least one of a group of afirst target potential and a second target potential and a group of athird target potential and a fourth target potential, the first targetpotential being higher than a potential of the first communication lineby a constant voltage, the second target potential being higher than apotential of the second communication line by the constant voltage, thethird target potential being lower than the potential of the firstcommunication line by the constant voltage, the fourth target potentialbeing lower than the potential of the second communication line by theconstant voltage, wherein: when the driving portion drives the firstcathode-side element part and the second anode-side element part, thedriving portion generates the differential voltage between the firstcommunication line and the second communication line based on theconstant voltage by at least one of inputting the second targetpotential to a control terminal of the first cathode-side element partand inputting the third target potential to a control terminal of thesecond anode-side element part; when the driving portion drives thesecond cathode-side element part and the first anode-side element part,the driving portion generates the differential voltage between the firstcommunication line and the second communication line based on theconstant voltage by at least one of inputting the first target potentialto a control terminal of the second cathode-side element part andinputting the fourth target potential to a control terminal of the firstanode-side element part; and the differential voltage generated when thedriving portion drives the first cathode-side element part and thesecond anode-side element part and the differential voltage generatedwhen the driving portion drives the second cathode-side element part andthe first anode-side element part have different polarities.
 2. Thetransmission device according to claim 1, wherein: each of the firstcathode-side element part, the second cathode-side element part, thefirst anode-side element part, and the second anode-side element partincludes one of a source follower circuit and an emitter followercircuit; the source follower circuit includes a field effect transistorwhose source is coupled with corresponding one of the firstcommunication line and the second communication line; and the emitterfollower circuit includes a bipolar transistor whose emitter is coupledwith corresponding one of the first communication line and the secondcommunication line.
 3. The transmission device according to claim 1,wherein the target potential generating portion includes: a constantcurrent circuit coupled between the cathode-side power supply line andthe anode-side power supply line; a current mirror circuit; a firstcurrent channel and a second current channel coupled with the constantcurrent circuit through the current mirror circuit; a first pair ofdiodes disposed on the first current channel and coupled with each otherthrough a first junction point; and a second pair of diodes disposed onthe second current channel and coupled with each other through a secondjunction point, wherein the first communication line is coupled to thefirst junction point and the second communication line is coupled to thesecond junction point so that each of the target potentials generatedwith reference to one of the potential of the first communication lineand the second communication line is generated on an opposite side ofcorresponding one of the diodes from the one of the first communicationline and the second communication line.
 4. The transmission deviceaccording to claim 1, further comprising: a first upper-limit controlportion coupled in parallel with the first cathode-side element part,the first upper-limit control portion controlling the potential of thefirst communication line to be lower than or equal to an upper limitpotential that is determined based on a potential of the cathode-sidepower supply line; a second upper limit control portion coupled inparallel with the second cathode-side element part, the secondupper-limit control portion controlling the potential of the secondcommunication line to be lower than or equal to an upper limit potentialthat is determined based on the potential of the cathode-side powersupply line; a first lower-limit control portion coupled in parallelwith the first anode-side element part, the first lower-limit controlportion controlling the potential of the first communication line to behigher than or equal to a lower limit potential that is determined basedon a potential of the anode-side power supply line; and a secondlower-limit control portion coupled in parallel with the secondanode-side element part, the second lower-limit control portioncontrolling the potential of the second communication line to be higherthan or equal to a lower limit potential that is determined based on thepotential of the anode-side power supply line.